1. Field of the Invention
The present invention generally relates to a transport belt drive control device, an image forming device, and a transport belt drive control method. More specifically, the present invention relates to the a transport belt drive control device, an image forming device, and a transport belt drive control method, which controls drive of a transport belt for transporting a recording medium in an image forming device of an ink jet recording method.
2. Description of the Related Art
Generally, in an image forming devices, such as an ink-jet printer, an image formation is performed on a recording medium (for example, paper) by a width equivalent to the nozzle width of the ink jet head, and thereafter the recording medium is transported in the sub-scanning direction and stopped by controlling drive of the transport belt. This procedure is repeatedly carried out, and finally a desired image is formed on the recording medium of one sheet.
In recent years, with the improvements of light resistance of ink and degradation effects of time on ink, the ink is changed from the dye type to the pigment type, and, moreover, the use of high-viscosity ink is progressing.
Although the blotting of ink to the recording medium is decreased sharply by the use of high-viscosity ink, poor accuracy of the positions of ink drops discharged to the recording medium causes the appearance of the printed image to deteriorate (white stripe, black stripe, banding). Especially, the contribution of the stop position accuracy at the time of transporting the recording medium in the sub-scanning direction is large, the increase in the stop position accuracy has been the indispensable technical object of the image forming device.
Conventionally, for the recording medium transport mechanism in the image forming device of ink jet recording method, the transport method utilizing a conveyance roller or a transport belt has been commonly used. And the method of controlling the feed amount of the conveyance roller or the transport belt is that a cord wheel is disposed on a conveyance roller shaft, and an output of an encoder sensor indicating a movement of the cord wheel is read to control the feed amount of the roller or the belt.
There are several known methods of controlling the feed amount of the recording medium. For example, refer to Japanese Laid-Open Patent Application No. 07-243870.
FIG. 1 shows the composition of a conventional image forming device in which a feed amount control of the transport belt is performed to control the feed amount of a recording medium laid on the transport belt.
In the conventional image forming device of FIG. 1, the feed amount control of the transport belt is performed by reading an output of the indirect encoder sensor 225 which indicates a movement of the rotary scale 226 disposed on the circumference of the cord wheel 233 which is rotated by the drive motor 221.
For example, when the control of the belt feed amount equivalent to 1000 pulses is performed using a computation unit, such as a CPU, the feed amount control of the transport belt is performed as follow. The feeding of the transport belt by the drive motor 221 is continued until the counting of the rotary scale equivalent to 1000 pulses using the output of the indirect encoder sensor 225 is completed, and the electric supply to the drive motor 221 is stopped upon completion of the counting so that the movement of the transport belt 222 is stopped.
In the conventional image forming device of FIG. 1, the drive motor 221 and the cord wheel 233 are connected via the belt conveyance roller 38 by the belt 232. The left-hand end of the transport belt 222 is wound on the conveyance roller 38, and the right-hand end of the transport belt 222 is wound on the driven roller 231.
The feed and stop control of the transport belt 222 is performed by counting the rotary scale 226 disposed on the circumference of the cord wheel 233, using the output of the indirect encoder sensor 225. However, in this case, if a misalignment between the center of the cord wheel 233 and the center of the revolving shaft exists, then the counting of the same count value does not result in the same feed amount of the transport belt. Namely, a difference will arise in the feed amount of the transport belt.
FIG. 2 is a diagram for explaining the problem of the conventional image forming device. For the sake of convenience of explanation, an extreme example is shown in FIG. 2.
As shown in FIG. 2, suppose that a misalignment between the true center X2 of rotation of the cord wheel 233 and the center X1 of rotation of the actually installed shaft has arisen. In this case, it is clear that a difference arises in the feed amount of the transport belt even if the same count value (for example, 1000 pulses) is counted for the rotary scale. Apart from an installation error as in the above example, a thermal expansion of the cord wheel 233 according to environmental conditions and an error of the molded thickness of the transport belt 222 from a given design thickness may be the factors affecting the accuracy of the feed amount of the transport belt. In such case, even if the counting of the same count value is performed by using the output of the indirect encoder sensor 225, it is difficult to control the feed amount of the transport belt 222 to a fixed amount with good accuracy.